A line of five feet, each of which is an iamb, that is to say, each of which is a ti-tum. As opposed to a tum-ti.

Ti-tum ti-tum ti-tum ti-tum ti-tum

In fact, if you analyse a passage of blank verse, that is to say poetry written in unrhymed iambic pentameters, you will find very few lines that conform precisely to this pattern:

The woods decay, the woods decay and fall

Nobody could read this line of Tennyson (from "Tithonus") without reading it metrically, this is to say, placing emphasis on the second, fourth, sixth, eighth and tenth syllables. But while this is an example of a regular iambic pentameter, it is not common, in good poetry, to find such perfect lines, or to find several of them in succession.

The iambic pentameter owes its pre-eminence in English poetry to its genius for variation. Good blank verse does not sound like a series of identically measured lines. It sounds like a series of subtle variations on the same theme.

The key to the historic success of this line is its being neither too long nor too short. If it were any longer, the reader would have to emphasise the metre a little more, in order to assert control of the line. You can hear this need asserting itself in English poetry written in the longer classical line, the hexameter. You have to assert the metre, otherwise you will get lost. But if the iambic pentameter is properly written, you shouldn't have any difficulty understanding how it goes. The poet should have written it so that it comes trippingly off the tongue.

Again, if the poetic line is shorter than the 10 syllables of the iambic pentameter, what happens is that the metre asserts itself willy-nilly, because there is less room for variation. This is not a fault of shorter lines, it is merely a characteristic of them. You may choose a shorter line precisely in order to enjoy this extra degree of assertiveness. But in choosing the more assertive line you have to bear in mind the length of the poem. You have to decide whether you can keep your more assertive line going for more than a page or two.

The iambic pentameter is ideal for a long poem because of this capacity for each variation.

The woods decay, the woods decay and fall, The vapours weep their burthen to the ground

Once again, in the second line of the poem, it would be hard to read the words wrongly. And yet, if you read the words as they feel they should sound, you will automatically skip the emphasis on the eighth syllable. The metrical template - ti-tum ti-tum ti-tum ti-tum ti-tum - would lead you to expect an emphasis on the word "to":

The vapours weep their burthen to the ground

But that would sound idiotic. So one of the five metrical accents has been dropped. In the next line, by way of further variation, we find that there are six stresses:

The woods decay, the woods decay and fall, The vapours weep their burthen to the ground, Man comes and tills the field and lies beneath

You cannot read line three without placing emphasis on both of the first two words - and this is an effect perfectly consonant with the meaning of the poem. Man has arrived on the scene, and has disrupted things by his arrival. "Man comes" - if you emphasise both words equally, which is what you are being asked to do, you are turning the first foot of the line into what is called a spondee.

An iamb goes: ti-tum.

A trochee goes: tum-ti.

A spondee goes: tum! tum!

But spondees are much rarer than iambs and trochees in English verse, because it normally happens that if you put two words together, or two syllables together, one of them will attract more weight, more emphasis, than the other. In other words, most so-called spondees can be read as either iambs or trochees.

These technical terms - iamb, trochee, spondee - which come to us from classical metrics, are used as a matter of convenience, but they can give a false impression of rigour when we use them in an English context. In English poetry there is no such thing as a regular use of the spondee - it is more like a specific local effect. You couldn't write a whole poem in spondees - you couldn't even write a single line. But you can write immense poems in iambics.

The woods decay, the woods decay and fall, The vapours weep their burthen to the ground, Man comes and tills the field and lies beneath, And after many a summer dies the swan.

In the fourth line Tennyson could easily have avoided the extra syllable - the line has 11 syllables -by making the summers into a more conventional plural:

And after many summers dies the swan.

But he does not do this. Clearly he prefers the idiomatic flavour of "many a". But there is another point here: the irregularity he has introduced is not a mistake. It is another variant, and in this case a very gentle variant, on the basic pattern. You could argue, indeed, that it hardly counts as a variant, since most people would run the last syllable of "many" together with "a", making one syllable. They would elide the syllables. They would create an elision.

· This is an edited extract from James Fenton's book An Introduction to English Poetry (Viking, £14.99)